You can use whatever size you can fit under the van.
The most common is a 10" Diameter x 32" Long, which has an L.P. Capacity of about 7.5 gal. (W.C.- water capacity- is 10 gal.)
Next (in my experience) is a 10" x 40" Long, which has an L.P. Capacity of about 10 gal. (W.C. is 12.5 gal.)
I'm not sure what the next larger size is in the 10" diameter line of tanks.
The next larger diameter is 12", but many vans don't have a cavity that allows a 12" diameter tank, so they're not commonly stocked in our industry.
10 x 40 is about as long as I've seen in a carpet cleaning application, even though we have one currently that's 12" x 48" for a box truck application.